Multiple-access channel protocols are applicable to data and voice channels.
Satellite Data Channels:
Typical multi-access protocols for a satellite data channels are derived from the basic ALOHA or slotted ALOHA (S-ALOHA), in which a mobile user (MU) spontaneously sends data (asynchronous or in time slots) on the multi-access channel and then listens to the echo of the transmission from the satellite to decide whether a collision occurred. In a half-duplex system, the MU hears the actual relay echo of its signal from the satellite, while in a full duplex frequency system, the MU hears the result of its transmission into uplink multi-access channel as a uplink channel status "echo message" written to data of the downlink channel. Depending upon the length of data transmissions by the MUs, the protocol might also require an MU to determine the current state of the uplink channel (busy or idle) and delay transmission until the channel is idle. Over satellite channels, data transmission protocols derived from ALOHA are inherently inefficient because of the long round trip times associated with satellite channels (roughly 0.25 seconds for GEO satellites).
Satellite Voice Channels:
The typical multi-access (i.e., party line) protocol for a voice channel is a "push-to-talk" protocol. In this protocol, an MU listens to the multi-access channel on an ongoing basis. When the MU has a need to talk, it waits until the channel is idle (no talking), pushes a button on the transceiver (which seizes the channel), and proceeds to talk. If two MUs "push-to-talk" within the round trip delay time of the channel, a voice collision occurs, and both MUs must start over. Over a satellite channel, "push-to-talk" is an inherently unwieldy and inefficient protocol because the long round-trip delay time leads to may voice collisions.